Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with your doctor.
As you mention, soy has estrogen-like components, and hormones can have certain effects on seizures. In menstruating women, in general, seizures occur when estrogen levels are high and progresterone levels are low. To my knowledge, there has not be extensive research on soy as a trigger for seizures (though the internet is full with this type of information, well designed clinical research studies are not available to my knowlege in this regard). If you have clearly noticed this as a trigger, then avoiding soy-containing products may be the best way to reduce the triggering of seizures. Other factors that reduce seizure threshold, such as sleep deprivation and excessive alcohol consumption, should also be avoided.
It may benefit you to be evaluated by an epilepsy specialist (a neurologist with specific training in epilepsy) who is also specialized in women health issues. Such as staff are available in various epilepsy centers.
Thank you for this opportunity to answer your questions, I hope you find the information I have provided useful, good luck.